380 NOTES ON rLYMOUTII SPONGES. 



and massively " spongy," structure opens up a totally different series of 

 needs and adaptations, not comparable with the delicate simplicity of 

 the Sycon. One point only may be noted ; that for Halichondria with 

 its few wide oscula, the difficulty with the contained bubble is slight, 

 and when the oscula point upwards it will escape at once. I am 

 somewhat of opinion that the hairy coat of ^S*. ciliatum may assist it in 

 another way to retain moisture, when, in its intermediate situation, 

 hanging under sheltering masses of damp seaweed ; it is worth noting 

 that S. lingua, from Haeckel's description, has no cortex, but a very 

 long fur. 



It would involve far more space and detailed discussion than are here 

 convenient to endeavour to assign the exact importance of the few facts 

 above narrated, nor until associated with many parallel observations is 

 it worth while. The subservience of a " marked species characteristic " 

 to outward circumstances, shown in the partly cylindrical form of tide- 

 pool specimens, may be due to the fact that only here such varieties can 

 survive, may indicate a power of individual adaptation. Probably it 

 means merely that mobility, never exercised, is lost; and that the 

 spicules which are never called on to slide over one another become 

 locked and plaited to the ligidity of other Sycons. I have before now 

 endeavoured to show * that the definite series of changes in canal- 

 system and outward form, with which homoplasy presents us again and 

 again in every group of Porifera, bring definite increasing mechanical 

 advantage. Here I have attempted to argue that the most definitely 

 characterised common species of sponge has the most definite use for its 

 species characters. I hope later to be able to show, in the case of 

 Reniera, that the minute spicular changes which fill our classifications, 

 and to which it appears impossible to ascribe utilitarian value, are not 

 characteristic of species, but merely the direct consequence in the 

 individual of some altered physical conditions of the nutrient medium. 



(2) Halichondria panicea ; Suberites domunculus : 



A'ariation and Meta^ipy. 



The specimen of //. ixinicca given me from Exmouth {vide p. 317) 

 differs markedly from a Plymouth specimen, or from Bowerbank's 

 figures, in having the interior skeleton far looser and more fibrous in 

 character. The Exmouth specimen shows, even in the innermost mass, 

 numerous well-marked bundles, three or four spicules in thickness, 

 branching, Ijut ha\'ing a general tendency to parallelism. The Plymouth 

 specimen shows the confused skeleton recognised as characteristic by 

 all authors; and the far more numerous spicules form, in the interior of 



* Loc. ciL, p. IS, and Froc. li. S., vol. 64, ji. Gl. 



1 



